Image of hand shows progress toward proton radiography

In this image based on proton radiography of a hand phantom, colors represent the summed-up proton-stopping power in terms of water-equivalent thickness, showing the varying thickness of the hand and clear structural details. (Images courtesy of H. Sadrozinski)Proton radiograph of a hand phantom.

Researchers developing a new medical imaging technology that uses protons instead of x-rays presented the first proton radiographic image of a hand this week at a medical imaging conference in Southern California.

The image echoes one of the most famous images in the history of medical science: Wilhelm Roentgen's 1895 x-ray image of his wife's hand titled "Hand mit Ringen" ("hand with ring"). Nowadays, experimenting on one's spouse is frowned upon, so the proton imaging team used a radiographic hand phantom (an anatomical model of a hand with the same radiographic properties). They imaged it with protons from the medical proton synchrotron at Loma Linda University Medical Center (LLUMC).

"This first image demonstrates the new promise of proton imaging, which is now within reach of becoming a new, potentially low-dose medical imaging modality," said Hartmut Sadrozinski, a research physicist at the Santa Cruz Institute for Particle Physics (SCIPP) at UC Santa Cruz. "Our ultimate goal is to do proton computed tomography and reconstruct the images in three dimensions, like an x-ray CT scan."

The interdisciplinary team working on this project includes physicists and students at UC Santa Cruz, medical researchers and doctors at Loma Linda University, and computer scientists at California State University, San Bernardino. They presented their findings at the 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference in Anaheim in a poster presentation on October 29 and in an invited talk by Dr. Reinhard Schulte from LLUMC, "A status update on proton imaging for applications in medicine," on October 30.

The proton imaging project is motivated by the increasing use of proton beams in cancer therapy. In proton therapy, powerful doses of radiation can be delivered directly to a tumor with little damage to surrounding healthy tissue. To do this, an image of the proton "stopping power" of the tissues is needed to guide the cancer treatment planning. This is achieved today by essentially translating x-ray images into proton stopping-power images, which compromises the accuracy of the images for proton treatment planning.

"The goal of our research is to generate these proton CT images directly, allowing more precise treatment planning," Sadrozinski said.

Like Roentgen's x-ray image, the proton image of the hand phantom clearly shows details of the bone structure. Unlike x-rays, however, protons also show the soft tissue of the hand in more detail, which reflects differences in how the two forms of radiation interact with tissues. Whereas x-rays get absorbed preferentially by bones and show them much more clearly than the surrounding soft tissue, protons get slowed down or stopped by bone only 50 percent more than by soft tissue.

In addition to the progress in image reconstruction reported at the meeting, the researchers are developing innovations in detector technology that they will incorporate into future prototypes of the proton CT scanner.

The work was performed by UCSC graduate student Tia Plautz, undergraduate students Celeste Leary, Andrew Plumb, and David Steinberg, and LLUMC researcher Robert F. Hurley (a graduate of the UCSC Physics Department). Grants from the National Institutes of Health and the National Science Foundation fund the work at the Santa Cruz Institute for Particle Physics, Loma Linda University and California State University San Bernardino. The researchers used mathematical algorithms and computer software developed in part by researchers at LLUMC, the University of Haifa (Israel), the University of Wollongong (Australia), and Stanford University (U.S.). The proton imaging detectors were built at UC Santa Cruz and Northern Illinois University with support from the Department of Defense and the Department of Radiation Medicine at LLUMC.

This research in proton CT is supported by the National Institute of Biomedical Imaging and Bioengineering (award number R01EB013118), the U.S. Department of Defense Prostate Cancer Research Program (award number W81XWH-12-1-0122), and the United States-Israel Binational Science Foundation (award number 2009012).